Variable angle spectroscopic ellipsometry (VASE) technique is used to measure the optical constants of InGaAs/GaAs multiple quantum well (MQW) structures for the purpose of designing tunable optical modulators. The VASE measurements also include field-induced changes in index of refraction and absorption. These measured changes in MQWs are in agreement with theoretical computations. In addition, the design and simulated performance of an InGaAs/GaAs MQWs Fabry–Perot modulator using the VASE data is presented. ©1997 American Institute of Physics.